There are many processes in which a directly contacting probe or the like cannot be employed to monitor changes of position, for example because of a hostile environment or the aggressive nature of the material being monitored. Non-contacting sensors must then be used, such as devices responsive to magnetic fields.
Non-contacting liquid level indicators have been described employing a vertical array of magnetic switches, as in the examples of GB 99786 and U.S. Pat. Nos. 4,627,283 and 4,589,282, in which a magnet carried by a float operates the switches without contacting them. Such switches have an on-off operation so that they provide stepped measurements, but that is not necessarily a disadvantage if the steps are sufficiently small. In the last-mentioned U.S. Pat. No. 4,589,282 in particular, there is proposed the use of Hall-effect switches, which provide relatively compact and convenient switching devices with a small power consumption.
These earlier proposals are limited in respect of range and/or accuracy of measurement, however. It will be understood, that the incremented signal change between successive steps must be sufficient to be unaffected by circuit noise so that it soon becomes inconvenient to cope with the increasing signal voltage range if it is required to have an indicator with a very large number of steps, whether for increased accuracy or for greater range of measurement. In the apparatus of U.S. Pat. No. 4,589,282 a vernier-like arrangement of switches and magnets is proposed to increase the accuracy of measurement but that requires complex circuitry which itself poses a practical limit to the maximum number of steps or graduations over the range of measurement.